Abstract: Pinus sylvestris var. mongolica is a major species for forest culture in northern China. The quality of its seedlings is crucial for successful afforestation. Trichoderma spp. can effectively promote the growth of crops, induce systematic resistance of plants and increase seed germination and root and shoot biomass. However, most studies of Trichoderma spp. promotion have focused on crops and only very few on forest tree seedlings. In order to investigate the effects of T. virens on the growth, physiological and biochemical parameters of P. sylvestris var. mongolica seedlings, we incubated 1-year-old seedlings with T. virens T43, by drilling and root irrigation and examined dynamically the activities of catalase (CAT), peroxidase (POD) , superoxide dismutase (SOD), as well as the amounts of soluble protein and soluble sugar. At the end of the growing season, we measured the biomass of T43-inoculated T. virens seedlings. Our results indicated that T. virens T43-inoculated seedlings had higher enzyme activity than the non-inoculated control (CK) seedlings throughout the entire trial, with a steady increase for CAT and POD and a fluctuating upward trend for SOD. Similarly, T43 inoculation resulted in higher amounts of soluble protein and soluble sugar than those in the CK throughout the trial, with an immediate increase in enzyme activities after inoculation, followed by a decline to a steady state at a lower level for both treated and non-treated seedlings. The inoculated seedlings also had increases in height (36.7%), collar diameter (22.2%), dry weight (50%) and fresh weight (21.7%) three months after inoculation compared to the control. Our study suggests that it is feasible to use T. virens T43 to promote the growth and improve the quality of P. sylvestris var. mongolica seedlings.